cleanup comments about integer types

[jeffhammond]

Some of these commends are no longer applicable/accurate...

/* MPI_Aint is defined to be intptr_t (or equivalent to it, if compiler support is absent).
 * The only acceptable alternative to intptr_t is the C89 type equivalent to it. */
#if !defined(MPI_ABI_Aint)
#define MPI_ABI_Aint intptr_t
#endif
typedef MPI_ABI_Aint MPI_Aint;
#undef  MPI_ABI_Aint

/* MPI_Offset will be 64b on all relevant systems.
 * We allow for MPI implementations supporting for 128b filesystems. */
#if !defined(MPI_ABI_Offset)
#define MPI_ABI_Offset int64_t
#endif
typedef MPI_ABI_Offset MPI_Offset;
#undef  MPI_ABI_Offset

/* MPI_Count must be large enough to hold the larger of MPI_Aint and MPI_Offset.
 * Platforms where MPI_Aint is larger than MPI_Offset are extremely rare. */
#if !defined(MPI_ABI_Count)
#define MPI_ABI_Count MPI_Offset
#endif
typedef MPI_ABI_Count MPI_Count;
#undef  MPI_ABI_Count